In order for jack to work, your user needs to be [[Users and Groups#Group management|added]] to the {{ic|audio}} group for direct access to hardware.

−

JACK2 is currently available in the [community] and can be installed from there using

−

pacman -S jack2

−

or

−

pacman -S jack2-dbus

−

Jack2 without [[D-Bus]] executes similarly to Jack1. With D-Bus, control is done using the jack_control utility. Run `jack_control` to see what it does. The important commands are listed below:

+

===JACK2===

+

'''JACK2''' is rewritten explicitly towards multiprocessor hardware. Install it with {{Pkg|jack2}}, available from the [[official repositories]].

+

+

Also available is also JACK2 with [[D-Bus]]. It can be installed with {{Pkg|jack2-dbus}}. It is controlled by the {{ic|jack_control}} utility. The important commands are listed below:

jack_control start - starts the jack server

jack_control start - starts the jack server

jack_control stop - stops the jack server

jack_control stop - stops the jack server

Line 17:

Line 16:

===JACK===

===JACK===

−

Alternatively, there is the normal version from the extra repository, install it with:

+

Alternatively, there is the older '''JACK''', installable with {{pkg|jack}}, available from the [[official repositories]].

−

pacman -S jack

−

−

==Basic Configuration==

===GUI===

===GUI===

+

No matter which JACK edition you are using, you will want a GUI configurator. Probably the de facto standard right now, is QjackCtl, installable with the package {{Pkg|qjackctl}}, available in the [[official repositories]].

−

No matter which JACK edition you are using, you will want a GUI configurator. Probably the de facto standard right now, is qjackctl:

+

==Basic Configuration==

−

pacman -S qjackctl

+

===Overview===

−

===Setup===

+

[http://w3.linux-magazine.com/issue/67/JACK_Audio_Server.pdf This Linux Magazine article] is a very good general overview, although do not worry about manual compilations, quite a few JACK tools work right off the wire now, ''after'' JACK is configured correctly.

−

====Overview====

+

Most tutorials are advising a realtime kernel, which is quite helpful for live synthesis and FX; but for purposes of recording and editing it is not necessary, as long as you set up for non-realtime latencies -- 10-40+ ms (100-500+ ms for older hardware).

−

Here is a very good general overview, although do not worry about manual compilations, quite a few JACK tools work right off the wire now, ''after'' JACK is configured correctly.

+

The right configuration for your hardware and application needs, depends on several factors.

−

http://w3.linux-magazine.com/issue/67/JACK_Audio_Server.pdf

+

===A Shell-Based Example Setup===

−

Most tutorials are advising a realtime kernel, which is quite helpful for live synthesis and FX; but for purposes of recording and editing it is not necessary anymore, as long as you set up for non-realtime latencies -- 10-40+ ms, and on older hardware 100-500+ ms.

+

The D-Bus edition of JACK2 can make startup much easier. Formerly, we had to have QjackCtl start it for us, or use a daemonizer, or some other method. But using {{Pkg|jack2-dbus}}, we can easily start and configure it via a shell script.

−

The right configuration for your hardware and application needs, depends on several factors.

+

Create a shell script that can be executed at X login:

−

====A Shell-Based Example Setup====

+

{{hc|start_jack.sh|

+

#!/bin/bash

−

The D-Bus edition of JACK2 can make startup much easier. Formerly, we had to have qjackctl start it for us, or use a daemonizer, or some other method. But as long as jack2-dbus is installed and dbus is running (which is Arch standard now), we can create a shell script as follows, to be run at X login:

+

jack_control start

+

sudo schedtool -R -p 20 `pidof jackdbus`

+

jack_control eps realtime true

+

jack_control ds alsa

+

jack_control dps device hw:HD2

+

jack_control dps rate 48000

+

jack_control dps nperiods 2

+

jack_control dps period 64

+

sleep 10

+

/usr/bin/a2jmidid -e &

+

sleep 10

+

sudo schedtool -R -p 20 `pidof a2jmidid`

+

qjackctl &

+

sleep 10

+

sudo schedtool -R -p 20 `pidof qjackctl`

+

qmidiroute /home/jeb/All2MIDI1.qmr &

+

sleep 10

+

sudo schedtool -R -p 20 `pidof qmidiroute`

+

yoshimi -S &

+

sleep 10

+

sudo schedtool -R -p 20 `pidof yoshimi`

+

}}

−

#!/bin/bash

+

The above will start a complete realtime JACK live-synthesis setup, integrating several tools. Details of each line follow. When discovering your own best configuration, it is helpful to do trial and error using QjackCtl's GUI with a non-D-Bus JACK2 version.

−

jack_control start

−

sudo schedtool -R -p 20 `pidof jackdbus`

−

jack_control eps realtime true

−

jack_control ds alsa

−

jack_control dps device hw:HD2

−

jack_control dps rate 48000

−

jack_control dps nperiods 2

−

jack_control dps period 64

−

sleep 10

−

/usr/bin/a2jmidid -e &

−

sleep 10

−

sudo schedtool -R -p 20 `pidof a2jmidid`

−

qjackctl &

−

sleep 10

−

sudo schedtool -R -p 20 `pidof qjackctl`

−

qmidiroute /home/jeb/All2MIDI1.qmr &

−

sleep 10

−

sudo schedtool -R -p 20 `pidof qmidiroute`

−

yoshimi -S &

−

sleep 10

−

sudo schedtool -R -p 20 `pidof yoshimi`

−

The above will start a complete realtime JACK live-synthesis setup, integrating several tools. Details of each line follow. When discovering your own best configuration, it is helpful to do trial and error using QJackCTL's GUI with a non-D-Bus JACK2 version.

+

====Details of the Shell-Based Example Setup====

−

−

=====Details of the Shell-Based Example Setup=====

jack_control start

jack_control start

Line 86:

Line 82:

Sets JACK to use 2 periods. 2 is right for motherboard, PCI, PCI-X, etc.; 3 for USB.

Sets JACK to use 2 periods. 2 is right for motherboard, PCI, PCI-X, etc.; 3 for USB.

jack_control dps period 64

jack_control dps period 64

−

Sets JACK to use 64 periods per frame. Lower is less latency, but the setting in this script gives 2.67 ms latency, which is nicely low without putting too much stress on the particular hardware this example was built for. If a USB sound system were in use it might be good to try 32. Anything less than 3-4 ms should be fine for realtime synthesis and/or FX, 5 ms is the smallest a human being can detect. There are many cases of perfect-storm-gorgeous hardware which can handle 1 ms latency without stressing the CPU, but definitely this is not always the case! QJackCTL will tell you how you are doing; at no-load, which means no clients attached, you will want a max of 3-5% CPU usage, and if you cannot get that without xruns (the red numbers which mean the system cannot keep up with the demands), you will have to improve your hardware. There are many inexpensive USB sound systems which produce very good quality at very low latency if the USB is good on the motherboard, but not all.

+

Sets JACK to use 64 periods per frame. Lower is less latency, but the setting in this script gives 2.67 ms latency, which is nicely low without putting too much stress on the particular hardware this example was built for. If a USB sound system were in use it might be good to try 32. Anything less than 3-4 ms should be fine for realtime synthesis and/or FX, 5 ms is the smallest a human being can detect. There are many cases of perfect-storm-gorgeous hardware which can handle 1 ms latency without stressing the CPU, but definitely this is not always the case! QjackCtl will tell you how you are doing; at no-load, which means no clients attached, you will want a max of 3-5% CPU usage, and if you cannot get that without xruns (the red numbers which mean the system cannot keep up with the demands), you will have to improve your hardware. There are many inexpensive USB sound systems which produce very good quality at very low latency if the USB is good on the motherboard, but not all.

sleep 10

sleep 10

Wait for the above to settle.

Wait for the above to settle.

Line 96:

Line 92:

Set a2jmidid to realtime scheduling in the Linux kernel.

Set a2jmidid to realtime scheduling in the Linux kernel.

qjackctl &

qjackctl &

−

Load QJackCTL. GUI configuration tells it to run in the system tray. It will pick up the JACK session started by D-Bus just fine, and very smoothly too. It maintains the patchbay, the connections between these applications and any other JACK-enabled apps to be started manually. The patchbay is set up using manual GUI, but connections pre-configured in the patchbay are automatically created by qjackctl itself when apps are started.

+

Load QjackCtl. GUI configuration tells it to run in the system tray. It will pick up the JACK session started by D-Bus just fine, and very smoothly too. It maintains the patchbay, the connections between these applications and any other JACK-enabled apps to be started manually. The patchbay is set up using manual GUI, but connections pre-configured in the patchbay are automatically created by QjackCtl itself when apps are started.

sleep 10

sleep 10

Wait for the above to settle.

Wait for the above to settle.

Line 114:

Line 110:

Set Yoshimi to realtime scheduling in the Linux kernel.

Set Yoshimi to realtime scheduling in the Linux kernel.

−

With all of the above in a script run at logon, and with the QJackCtl patchbay set correctly, all we have to do is plug the PC/laptop into a MIDI keyboard using a USB-to-MIDI adapter, or simply the USB-in MIDI capability of many modern keyboards, and you are ready to play!

+

With all of the above in a script run at logon, and with the QjackCtl patchbay set correctly, all we have to do is plug the PC/laptop into a MIDI keyboard using a USB-to-MIDI adapter, or simply the USB-in MIDI capability of many modern keyboards, and you are ready to play!

−

The essence of QJackCtl is described fairly well in [http://www.linuxjournal.com/article/8354 this article.]

+

The essence of QjackCtl is described fairly well in [http://www.linuxjournal.com/article/8354 this article.]

−

====A GUI-Based Example Setup====

+

===A GUI-Based Example Setup===

−

The shell-based example above, lays out in detail lots of things you may well need to know, and it does work well. If you want something simpler to handle, however, do this:

+

The shell-based example above, lays out in detail lots of things you may well need to know, and it does work well. If you want something much more GUI, however, do this:

−

* Install jack2-dbus.

+

* Install {{Pkg|jack2-dbus}}.

−

* In /etc/rc.local, add this line to the end:

+

* Copy {{ic|/etc/asound.conf}} to {{ic|/etc/asound.conf.ORIGINAL}}, and replace it with this:

−

sudo -u uname jack_start

+

pcm.pulse {

+

type pulse

+

}

+

ctl.pulse {

+

type pulse

+

}

+

pcm.!default {

+

type pulse

+

}

+

ctl.!default {

+

type pulse

+

}

−

where 'uname' is replaced with your username. This is the only non-GUI element of this setup; theoretically it's not necessary, but it can remove a whole lot of confusion in the login process, because this way Jack is running before login: for the rest of the audio software components to know clearly what they should do, it is best for Jack to run first. Do notice, though, that we're not configuring Jack yet. This means that if Jack was very badly configured at last boot, sound might not work until a good configuration is established (later in this list).

+

* Install {{Pkg|pulseaudio}}.

−

+

* Install {{Pkg|pulseaudio-alsa}}.

−

* Install qjackctl, and tell your GUI window/desktop system to run it at startup. Best to use a relatively low-resource-intensive window/desktop setup with a standard taskbar and tray area, e.g., [[MATE]].

+

* Install {{Pkg|qjackctl}}, and tell your GUI window/desktop system to run it at startup.

−

* Make sure qjackctl is told to:

+

* Make sure QjackCtl is told to:

−

** use the D-Bus interface,

+

** use the D-Bus interface,

−

** run at startup,

+

** run at startup,

** save its configuration to the default location,

** save its configuration to the default location,

** start the JACK audio server on application startup,

** start the JACK audio server on application startup,

** enable the system tray icon, and

** enable the system tray icon, and

−

** start minimized to ssytem tray.

+

** start minimized to sytem tray.

−

* Install pulseaudio.

−

* Install pulseaudio-alsa.

* Reboot.

* Reboot.

−

* After logging in, you will see QJackCtl in your system tray. Left-click on it.

+

* After logging in, you will see QjackCtl in your system tray. Left-click on it.

−

* Start tweaking in the QJackCtl GUI. The info embedded in the shell-script setup above may be of some help :-) As may be the info in [

+

* Start tweaking in the QjackCtl GUI. The info embedded in the shell-script setup above may be of some help :-) As may be the info in [http://www.linuxjournal.com/article/8354 this article]. Just remember that you have to get your latency down to less than 5ms for live tone production or filtration of any sort, or the delay will be obvious to player and listener alike.

−

The essence of QJackCtl is described fairly well in [http://www.linuxjournal.com/article/8354 this article.] Just remember that you have to get your latency down to less than 5ms for live tone production of any sort, or the delay will be obvious to player and listener alike.

+

* From the [[AUR]], install {{AUR|non-daw}}. One of the components of this package is called non-session-manager; it has the function of setting up "sessions": sets of other audio software items which Jack (through the QjackCtl patchbay or not!) will wire together. NSM can handle as many different sessions as you wish to set up; and as a result, it's all GUI, apart from the one rc.local edit in the beginning.

−

====More====

+

===More===

−

A lot more detail is in the [[Pro Audio]] page.

+

Yet more info is in the [[Pro Audio]] page.

== Jack for a multi-user system ==

== Jack for a multi-user system ==

+

{{Out of date|this needs to be updated for [[systemd]]}}

So, you have a decent multiuser system as it was designed more than 20 years ago, and now some developers decided that sound is only for a mono-user system... No I can not believe it !

So, you have a decent multiuser system as it was designed more than 20 years ago, and now some developers decided that sound is only for a mono-user system... No I can not believe it !

Line 227:

Line 233:

===Playing nice with ALSA===

===Playing nice with ALSA===

−

To allow Alsa programs to play while jack is running you must install the jack plugin for alsa:

+

To allow Alsa programs to play while jack is running you must install the jack plugin for alsa with {{Pkg|alsa-plugins}}.

−

{{bc|pacman -S alsa-plugins}}

And enable it by editing (or creating) /etc/asound.conf (system wide settings) to have these lines:

And enable it by editing (or creating) /etc/asound.conf (system wide settings) to have these lines:

Line 262:

Line 267:

You need not restart your computer or anything. Just edit the alsa config files, start up jack, and there you go...

You need not restart your computer or anything. Just edit the alsa config files, start up jack, and there you go...

−

Remember to start it as a '''user'''. If you start it with "jackd -d alsa" as user X, it will not work for user Y.

+

Remember to start it as a '''user'''. If you start it with {{ic|jackd}} -d alsa" as user X, it will not work for user Y.

=== gstreamer ===

=== gstreamer ===

Line 282:

Line 287:

autospawn = no

autospawn = no

−

''If you want both to play along, see: [[PulseAudio#PulseAudio_through_JACK_the_new_way]]''

+

''If you want both to play along, see: [[PulseAudio/Examples#PulseAudio through JACK the new new way]]''

==MIDI==

==MIDI==

Line 288:

Line 293:

JACK can handle one soundcard very well, and an arbitrary number of MIDI devices (connected e.g. via USB).

JACK can handle one soundcard very well, and an arbitrary number of MIDI devices (connected e.g. via USB).

If you start JACK and want to use a MIDI keyboard or a synthesizer or some other pure MIDI device, you have to start JACK with a proper soundcard (one that actually outputs or inputs PCM sound).

If you start JACK and want to use a MIDI keyboard or a synthesizer or some other pure MIDI device, you have to start JACK with a proper soundcard (one that actually outputs or inputs PCM sound).

−

As soon you have done that, you can connect the MIDI device. E.g. with {{pkg|QjackCtl}}, you click on the connect button and you will find your device listed under JACK-MIDI or ALSA-MIDI, depending on the driver.

+

As soon you have done that, you can connect the MIDI device. E.g. with QjackCtl ({{pkg|qjackctl}}), you click on the connect button and you will find your device listed under JACK-MIDI or ALSA-MIDI, depending on the driver.

−

For JACK-MIDI, you may want to set the '''MIDI Driver''' to '''seq''' or '''raw''' in {{pkg|QjackCtl}} ''Setup > Settings''. This should make your MIDI device appear under the ''MIDI'' tab. You can also change the name of the client (from a generic "midi_capture_1" to something more descriptive), if you enable ''Setup > Display > Enable client/port aliases'' and then ''Enable client/port aliases editing (rename)''.

+

For JACK-MIDI, you may want to set the '''MIDI Driver''' to '''seq''' or '''raw''' in QjackCtl ''Setup > Settings''. This should make your MIDI device appear under the ''MIDI'' tab. You can also change the name of the client (from a generic "midi_capture_1" to something more descriptive), if you enable ''Setup > Display > Enable client/port aliases'' and then ''Enable client/port aliases editing (rename)''.

−

For ALSA-MIDI, make sure to turn on '''Enable ALSA Sequencer support''' in {{pkg|QjackCtl}} ''Setup > Misc''. This will add the ''ALSA'' tab in {{pkg|QjackCtl}} ''Connect'' window where your MIDI controller will show up.

+

For ALSA-MIDI, make sure to turn on '''Enable ALSA Sequencer support''' in QjackCtl ''Setup > Misc''. This will add the ''ALSA'' tab in QjackCtl ''Connect'' window where your MIDI controller will show up.

−

For bridging ALSA-MIDI to JACK-MIDI, you may consider using {{AUR|a2jmidid}}. The following command will export all available ALSA MIDI ports to JACK MIDI ports:

+

For bridging ALSA-MIDI to JACK-MIDI, you may consider using a2jmidid ({{AUR|a2jmidid}}). The following command will export all available ALSA MIDI ports to JACK MIDI ports:

$ a2jmidid -e

$ a2jmidid -e

−

They will be visible in {{pkg|QjackCtl}} under the ''MIDI'' tab labelled "a2j" client.

+

They will be visible in QjackCtl under the ''MIDI'' tab labelled "a2j" client.

{{note|When connecting MIDI keyboard controllers in {{pkg|QjackCtl}}, make sure to ''Expand All'' first and connect the desired ''Output Ports'' (below the ''Readable Clients'') to the ''Input Ports'' (below the ''Writable Clients''). As a shortcut, if you select a writable client instead of individual ports as your destination, it should connect all its currently displayed output ports underneath.}}

+

{{note|When connecting MIDI keyboard controllers in QjackCtl, make sure to ''Expand All'' first and connect the desired ''Output Ports'' (below the ''Readable Clients'') to the ''Input Ports'' (below the ''Writable Clients''). As a shortcut, if you select a writable client instead of individual ports as your destination, it should connect all its currently displayed output ports underneath.}}

Still having the "Cannot allocate memory" and/or "Cannot connect to server socket err = No such file or directory" error(s) when pressing qjackctl's start button (assuming that you have package jack2-dbus installed) ?

JACK

GUI

No matter which JACK edition you are using, you will want a GUI configurator. Probably the de facto standard right now, is QjackCtl, installable with the package qjackctl, available in the official repositories.

Basic Configuration

Overview

This Linux Magazine article is a very good general overview, although do not worry about manual compilations, quite a few JACK tools work right off the wire now, after JACK is configured correctly.

Most tutorials are advising a realtime kernel, which is quite helpful for live synthesis and FX; but for purposes of recording and editing it is not necessary, as long as you set up for non-realtime latencies -- 10-40+ ms (100-500+ ms for older hardware).

The right configuration for your hardware and application needs, depends on several factors.

A Shell-Based Example Setup

The D-Bus edition of JACK2 can make startup much easier. Formerly, we had to have QjackCtl start it for us, or use a daemonizer, or some other method. But using jack2-dbus, we can easily start and configure it via a shell script.

The above will start a complete realtime JACK live-synthesis setup, integrating several tools. Details of each line follow. When discovering your own best configuration, it is helpful to do trial and error using QjackCtl's GUI with a non-D-Bus JACK2 version.

Sets JACK to use ALSA-compatible sound card named HD2. One can find the names with 'ls /proc/asound/cards'. Most ALSA tutorials and default configurations use card numbers, but this can get confusing when external MIDI devices are in use; names make it easier.

jack_control dps rate 48000

Sets JACK to use 48000 khz sampling. Happens to work very well with this card. Some cards only do 44100, many will go much higher. The higher you go, the lower your latency, but the better your card and your CPU has to be, and software has to support this as well.

jack_control dps nperiods 2

Sets JACK to use 2 periods. 2 is right for motherboard, PCI, PCI-X, etc.; 3 for USB.

jack_control dps period 64

Sets JACK to use 64 periods per frame. Lower is less latency, but the setting in this script gives 2.67 ms latency, which is nicely low without putting too much stress on the particular hardware this example was built for. If a USB sound system were in use it might be good to try 32. Anything less than 3-4 ms should be fine for realtime synthesis and/or FX, 5 ms is the smallest a human being can detect. There are many cases of perfect-storm-gorgeous hardware which can handle 1 ms latency without stressing the CPU, but definitely this is not always the case! QjackCtl will tell you how you are doing; at no-load, which means no clients attached, you will want a max of 3-5% CPU usage, and if you cannot get that without xruns (the red numbers which mean the system cannot keep up with the demands), you will have to improve your hardware. There are many inexpensive USB sound systems which produce very good quality at very low latency if the USB is good on the motherboard, but not all.

sleep 10

Wait for the above to settle.

/usr/bin/a2jmidid -e &

Start the ALSA-to-JACK MIDI bridge. Good for mixing in applications which take MIDI input through ALSA but not JACK.

sleep 10

Wait for the above to settle.

sudo schedtool -R -p 20 `pidof a2jmidid`

Set a2jmidid to realtime scheduling in the Linux kernel.

qjackctl &

Load QjackCtl. GUI configuration tells it to run in the system tray. It will pick up the JACK session started by D-Bus just fine, and very smoothly too. It maintains the patchbay, the connections between these applications and any other JACK-enabled apps to be started manually. The patchbay is set up using manual GUI, but connections pre-configured in the patchbay are automatically created by QjackCtl itself when apps are started.

sleep 10

Wait for the above to settle.

sudo schedtool -R -p 20 `pidof qjackctl`

Set qjackctl to realtime scheduling in the Linux kernel.

qmidiroute /home/username/All2MIDI1.qmr &

Load qmidiroute, loading a custom-created configuration file which will rewrite all MIDI events on all channels to channel 1. This is useful when plugging the PC into any keyboard anywhere -- no matter what the keyboard's channel defaults to, qmidiroute will send the signal to the synth on channel 1, where it needs it. qmidiroute is capable of very complex and useful configurations of many sorts, including multiple simultaneous translations, transpositions, signal type rewrites, etcetera.

sleep 10

Wait for the above to settle.

sudo schedtool -R -p 20 `pidof qmidiroute`

Set qmidiroute to realtime scheduling in the Linux kernel.

yoshimi -S &

Load the Yoshimi synthesizer, using the pre-saved default state.

sleep 10

Wait for the above to settle.

sudo schedtool -R -p 20 `pidof yoshimi`

Set Yoshimi to realtime scheduling in the Linux kernel.

With all of the above in a script run at logon, and with the QjackCtl patchbay set correctly, all we have to do is plug the PC/laptop into a MIDI keyboard using a USB-to-MIDI adapter, or simply the USB-in MIDI capability of many modern keyboards, and you are ready to play!

Install qjackctl, and tell your GUI window/desktop system to run it at startup.

Make sure QjackCtl is told to:

use the D-Bus interface,

run at startup,

save its configuration to the default location,

start the JACK audio server on application startup,

enable the system tray icon, and

start minimized to sytem tray.

Reboot.

After logging in, you will see QjackCtl in your system tray. Left-click on it.

Start tweaking in the QjackCtl GUI. The info embedded in the shell-script setup above may be of some help :-) As may be the info in this article. Just remember that you have to get your latency down to less than 5ms for live tone production or filtration of any sort, or the delay will be obvious to player and listener alike.

From the AUR, install non-dawAUR. One of the components of this package is called non-session-manager; it has the function of setting up "sessions": sets of other audio software items which Jack (through the QjackCtl patchbay or not!) will wire together. NSM can handle as many different sessions as you wish to set up; and as a result, it's all GUI, apart from the one rc.local edit in the beginning.

More

Jack for a multi-user system

So, you have a decent multiuser system as it was designed more than 20 years ago, and now some developers decided that sound is only for a mono-user system... No I can not believe it !

Warning: Before following the below instructions, please note that there is a security risk to any service running as root, and, more importantly, the developers for jack do not test it for running as root. In other words, it could eat your babies, data, or both

Fortunately some time ago someone convinced the developers to allow jack to run as a system wide daemon. Here is the procedure to follow:

Create a /etc/profile.d/jack.sh file containing:

export JACK_PROMISCUOUS_SERVER=""

Replace /etc/rc.d/jack-audio-connection-kit with the following content

PulseAudio

If you need to keep pulseaudio installed (in the event it is required by other packages, like gnome-settings-daemon), you may want to prevent it from spawning automatically with X and taking over from JACK.

Edit /etc/pulse/client.conf, uncomment "autospawn" and set it to "no":

MIDI

JACK can handle one soundcard very well, and an arbitrary number of MIDI devices (connected e.g. via USB).
If you start JACK and want to use a MIDI keyboard or a synthesizer or some other pure MIDI device, you have to start JACK with a proper soundcard (one that actually outputs or inputs PCM sound).
As soon you have done that, you can connect the MIDI device. E.g. with QjackCtl (qjackctl), you click on the connect button and you will find your device listed under JACK-MIDI or ALSA-MIDI, depending on the driver.

For JACK-MIDI, you may want to set the MIDI Driver to seq or raw in QjackCtl Setup > Settings. This should make your MIDI device appear under the MIDI tab. You can also change the name of the client (from a generic "midi_capture_1" to something more descriptive), if you enable Setup > Display > Enable client/port aliases and then Enable client/port aliases editing (rename).

For ALSA-MIDI, make sure to turn on Enable ALSA Sequencer support in QjackCtl Setup > Misc. This will add the ALSA tab in QjackCtl Connect window where your MIDI controller will show up.

For bridging ALSA-MIDI to JACK-MIDI, you may consider using a2jmidid (a2jmididAUR). The following command will export all available ALSA MIDI ports to JACK MIDI ports:

$ a2jmidid -e

They will be visible in QjackCtl under the MIDI tab labelled "a2j" client.
You can automate starting of a2jmidid by adding to QjackCtl Setup > Options > Execute script after Startup: /usr/bin/a2jmidid -e &

Note: When connecting MIDI keyboard controllers in QjackCtl, make sure to Expand All first and connect the desired Output Ports (below the Readable Clients) to the Input Ports (below the Writable Clients). As a shortcut, if you select a writable client instead of individual ports as your destination, it should connect all its currently displayed output ports underneath.

Q: What is the difference between JACK-MIDI and ALSA-MIDI?

A: The former has improved timing and sample accurate MIDI event alignment. It extends or may even replace the latter but at this point they both co-exist.

Troubleshooting

jack2-dbus and qjackctl errors

Still having the "Cannot allocate memory" and/or "Cannot connect to server socket err = No such file or directory" error(s) when pressing qjackctl's start button (assuming that you have package jack2-dbus installed) ?